An APK can be modified by an Android user to bypass the need for in-app purchases.
An acronym for “Android Package Kit”, a modded APK is one that has had its files altered and recompiled, often with special codes to modify or extend on some of its original functions. This kind of modification typically falls under the category of software tampering, as it constitutes a violation of copyright law protecting apps from being tampered with.
For example, if you wanted your app access to be permanent (i.e., without advertisements) or were looking for features such as time limits per move in chess games, you might go about trying to build a modded version that would satisfy those requirements.